Why can't I edit image for UV Map after joining Objects?
UV unwrap the mesh and select the texture you want it to be mapped to

Split the screen, turn it into the UV Image Editor.

Go back to the 3D view editor, select all the faces in Edit mode by pressingA

And back in the UV Image Edit header, you can select the image you will use as the texture.



Here is a tutorial

To texture paint it, you need to apply the texture. This is example is in Blender render, are you using cycles?